From the Gummi Bears:
An island that is impossible to reach or leave by ship because it is entirely surrounded by a waterfall. By which I mean, an island in the middle of a vast sinkhole in the ocean. A literal gulf several hundred yards wide into which the waters of the ocean unceasingly pour to an unknown and unseen depth. To peer into the void is to see an impossibly deep trench which ends only in blackness. The roar of the water rushing over the edge of the world drowns out any sound it might make hitting bottom. The island is not even with the ocean's level but sits somewhat within the sinkhole so that the view from land is at least halfway dominated by the site of the rushing wall of water.
This island could easily lie at the center of disk world (or anywhere, really; it just happens to sit within a hole in the disk).